U.S. patent application number 10/351693 was filed with the patent office on 2003-09-25 for method for preparation of metallic and ceramic foam products and products made.
This patent application is currently assigned to Advanced Ceramics Research, Inc.. Invention is credited to Artz, Gregory, Cipriani, Ronald A., Fulcher, Michael L., Lombardi, John L., Rigali, Mark J., Vaidyanathan, K. Ranji, Walish, Joseph.
Application Number | 20030180171 10/351693 |
Document ID | / |
Family ID | 31996486 |
Filed Date | 2003-09-25 |
United States Patent
Application |
20030180171 |
Kind Code |
A1 |
Artz, Gregory ; et
al. |
September 25, 2003 |
Method for preparation of metallic and ceramic foam products and
products made
Abstract
The present invention relates to the fabrication of low cost, in
situ, porous metallic, ceramic and cermet foam structures having
improved mechanical properties such as energy absorption and
specific stiffness. Methods of fabricating the structures from
compositions including ceramic and/or metallic powders are
provided. The flowable compositions also include an immiscible
phase that results in pores within the final structure.
Furthermore, the structures may be shaped to have external
porosity, such as with mesh-like structures.
Inventors: |
Artz, Gregory; (Tucson,
AZ) ; Vaidyanathan, K. Ranji; (Tucson, AZ) ;
Fulcher, Michael L.; (Tucson, AZ) ; Rigali, Mark
J.; (Tucson, AZ) ; Lombardi, John L.; (Tucson,
AZ) ; Walish, Joseph; (Miami, AZ) ; Cipriani,
Ronald A.; (Tucson, AZ) |
Correspondence
Address: |
BANNER & WITCOFF, LTD.
TEN SOUTH WACKER DRIVE
SUITE 3000
CHICAGO
IL
60606
US
|
Assignee: |
Advanced Ceramics Research,
Inc.
Tucson
AZ
|
Family ID: |
31996486 |
Appl. No.: |
10/351693 |
Filed: |
January 27, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10351693 |
Jan 27, 2003 |
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10094098 |
Mar 7, 2002 |
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6524522 |
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60274075 |
Mar 7, 2001 |
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Current U.S.
Class: |
419/2 |
Current CPC
Class: |
C04B 35/565 20130101;
B33Y 80/00 20141201; C04B 2111/00129 20130101; B33Y 10/00 20141201;
C04B 2235/6026 20130101; C04B 35/584 20130101; C04B 35/6365
20130101; B22F 2999/00 20130101; C04B 38/067 20130101; C04B 2235/94
20130101; C04B 35/638 20130101; C04B 35/63432 20130101; B22F 3/1121
20130101; B22F 10/10 20210101; B22F 1/10 20220101; B22F 3/1103
20130101; B22F 2003/1106 20130101; C04B 38/065 20130101; Y02P 10/25
20151101; B33Y 70/00 20141201; B22F 2998/10 20130101; C04B
2235/6022 20130101; C04B 2235/6021 20130101; B22F 2998/00 20130101;
B22F 3/227 20130101; B22F 3/1021 20130101; C04B 35/63424 20130101;
B22F 2003/1042 20130101; B22F 1/108 20220101; B22F 2998/00
20130101; B22F 3/227 20130101; B22F 2998/00 20130101; B22F 1/108
20220101; B22F 2998/10 20130101; B22F 3/22 20130101; B22F 7/002
20130101; B22F 3/1021 20130101; B22F 2999/00 20130101; B22F 3/1021
20130101; B22F 2201/02 20130101; B22F 2201/013 20130101; B22F
2999/00 20130101; B22F 10/10 20210101; B22F 3/22 20130101; C04B
38/065 20130101; C04B 35/565 20130101; C04B 35/584 20130101; C04B
38/0051 20130101; C04B 38/0054 20130101; C04B 38/0058 20130101;
C04B 38/0061 20130101; C04B 38/067 20130101; C04B 38/067 20130101;
C04B 35/565 20130101; C04B 38/069 20130101; C04B 40/0028 20130101;
B22F 2998/00 20130101; B22F 1/108 20220101 |
Class at
Publication: |
419/2 |
International
Class: |
B22F 003/00 |
Goverment Interests
[0002] The present invention was made with U.S. Government support
under grant Numbers DAAD19-00-C-0025 and DAAD19-01-C-0054 awarded
by the United States Army. Accordingly, the Government may have
certain rights in the invention described herein.
Claims
What is claimed is:
1. A method of fabricating a porous object comprising, in
combination: (a) mixing structure forming powder, binder, and a
pore forming compound to provide a flowable composition, wherein
the pore forming compound is generally immiscible with the
structure forming powder and binder; (b) gelling the flowable
composition in a mold to form a composite object; and (c) heating
the composite object to a temperature and for a time effective for
removing at least an amount of the pore forming compound to provide
a sintered object having internal porosity.
2. The method of claim 1 wherein the structure forming powder and
binder are mixed to form a generally uniform composition and
wherein the pore forming compound is subsequently mixed with the
composition.
3. The method of claim 1 wherein the pore forming compound is
present in the flowable composition as discrete droplets of regular
or irregular shape or combinations thereof.
4. The method of claim 1 wherein the structure forming powder is
selected from the group consisting of metal powder, ceramic powder,
cermet powder and combinations thereof.
5. The method of claim 1 wherein the pore forming compound is a
polymeric compound.
6. The method of claim 5 wherein the pore forming compound is
selected from the group consisting of polymethyl methacrylate,
ethyl vinyl acrylate, polyethylene ethacrylate,
poly-2-ethyl-2-oxazoline, polyethylene glycol, polystyrene,
microcellulose, butrylactone, hexanedioldiacrylate, and
combinations thereof.
7. The method of claim 1 wherein the flowable composition includes
one or more compounds for enhancing the processability of the
flowable composition.
8. The method of claim 1 wherein the flowable composition includes
one or more compounds for enhancing the material properties of the
sintered object.
9. A structure formed by the method of claim 1.
10. A method of fabricating a porous object comprising, in
combination: (a) mixing structure forming powder, binder, and
microspheres to provide a flowable composition; (b) gelling the
flowable composition in a mold to form a composite object; and (c)
heating the composite object to a temperature and for a time
effective for providing a sintered object having internal
porosity.
11. The method of claim 10 wherein the structure forming powder and
binder are mixed to form a generally uniform composition and
wherein the microspheres are subsequently mixed with the
composition.
12. The method of claim 10 wherein the structure forming powder is
selected from the group consisting of metal powder, ceramic powder,
cermet powder and combinations thereof.
13. The method of claim 10 wherein the microspheres are hollow
ceramic microspheres.
14. The method of claim 10 wherein the microspheres are polymeric
microspheres and wherein at least an amount of the polymeric
microspheres are removed during heating to provide pores within the
object.
15. The method of claim 10 wherein the flowable composition
includes one or more compounds for enhancing the processability of
the flowable composition.
16. The method of claim 10 wherein the flowable composition
includes one or more compounds for enhancing the material
properties of the sintered object.
17. A structure formed by the method of claim 10.
18. A method of fabricating a porous object comprising, in
combination: (a) mixing structure forming powder, binder, and
microspheres to provide an extrudable composition; (b) extruding
the extrudable composition through a dispensing head; (c)
depositing the extrudable composition onto a working surface in a
predetermined manner to form a composite object; and (d) heating
the composite object to a temperature and for a time effective for
providing a sintered object having internal porosity.
19. The method of claim 18 wherein the structure forming powder and
binder are mixed to form a generally uniform composition and
wherein the microspheres are subsequently mixed with the
composition.
20. The method of claim 18 wherein the structure forming powder is
selected from the group consisting of metal powder, ceramic powder,
cermet powder and combinations thereof.
21. The method of claim 18 wherein the microspheres are hollow
ceramic microspheres.
22. The method of claim 18 wherein the microspheres are polymeric
microspheres and wherein at least an amount of the polymeric
microspheres are removed during heating to provide pores within the
object.
23. The method of claim 18 wherein the flowable composition
includes one or more compounds for enhancing the processability of
the flowable composition.
24. The method of claim 18 wherein the flowable composition
includes one or more compounds for enhancing the material
properties of the sintered object.
25. The method of claim 18 wherein the extrudable composition is
deposited onto the working surface in one or more layers in a
matrix arrangement having structural porosity for increasing the
surface area of the composite structure.
26. A structure formed by the method of claim 18.
27. A method of fabricating a porous object comprising, in
combination: (a) mixing structure forming powder, binder, and a
pore forming compound to provide an extrudable composition, wherein
the pore forming compound is generally immiscible with the
structure forming powder and binder; (b) extruding the extrudable
composition through a dispensing head; (c) depositing the
extrudable composition onto a working surface in a predetermined
manner to form a composite object; and (d) heating the composite
object to a temperature and for a time effective for removing at
least an amount of the pore forming compound to provide a sintered
object having internal porosity.
28. The method of claim 27 wherein the structure forming powder and
binder are mixed to form a generally uniform composition and
wherein the pore forming compound is subsequently mixed with the
composition.
29. The method of claim 27 wherein the pore forming compound is
present in the flowable composition as discrete droplets of regular
or irregular shape or combinations thereof.
30. The method of claim 27 wherein the structure forming powder is
selected from the group consisting of metal powder, ceramic powder,
cermet powder and combinations thereof.
31. The method of claim 27 wherein the pore forming compound is a
polymeric compound.
32. The method of claim 27 wherein the pore forming compound is
selected from the group consisting of polymethyl methacrylate,
ethyl vinyl acrylate, polyethylene ethacrylate,
poly-2-ethyl-2-oxazoline, polyethylene glycol, polystyrene,
microcellulose, butrylactone, hexanedioldiacrylate, and
combinations thereof.
33. The method of claim 27 wherein the flowable composition
includes one or more compounds for enhancing the processability of
the flowable composition.
34. The method of claim 27 wherein the flowable composition
includes one or more compounds for enhancing the material
properties of the sintered object.
35. The method of claim 29 wherein the droplets of pore forming
compound become elongated and directionally aligned during
extrusion.
36. The method of claim 27 wherein the extrudable composition is
deposited onto the working surface in one or more layers in a
matrix arrangement having structural porosity for increasing the
surface area of the composite structure.
37. A structure formed by the method of claim 27.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of co-pending
U.S. application Ser. No. 10/094,098,filed Mar. 7, 2002, and
entitled "Method for Preparation of Metallic Foam Products and
Products Made." Further, this application is based on, and claims
the benefit of, co-pending U.S. Provisional Application No.
60/274,075, filed on Mar. 7, 2001, and entitled "Extrusion Freeform
Fabrication Method for Metallic Foam Components Having Oriented
Microstructures and Improved Mechanical Properties," which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to the fabrication of low
cost, in situ, metallic foam components having oriented
microstructures and improved mechanical properties such as energy
absorption and specific stiffness. The present invention also
relates to the fabrication of low cost, in situ, ceramic and,
cermet foam components with improved mechanical properties.
BACKGROUND OF INVENTION
[0004] Cellular materials have been available for decades, but new
opportunities for cellular materials are emerging. Novel
manufacturing approaches have beneficially affected performance and
cost. Additionally, higher levels of basic understanding about
mechanical, thermal and acoustic properties have been developed.
These provide an integrated pathway between manufacturing and
design.
[0005] Cellular materials have high stiffness and yield strength at
low density relative to other competing materials and systems. That
is, the cellular materials may be laminated between opposed sheets
of another material. This creates an opportunity for ultra-light
structures, with integrally bonded dense face sheets. In addition,
cellular materials have large compressive strains achievable at
nominally constant stress. Metallic foams provide for high-energy
absorption capacity which is advantageous in crash and blast
amelioration systems. These materials may be used effectively for
either cooling or heat exchange structures. Cellular materials
incorporated within a design to form sandwich skins can provide the
desired mechanical properties in a cost-effective manner as
compared to alternative structures.
[0006] One method of making metallic foams involves gas expansion
in foam casting. Another method for making metallic foams is based
on gas expansion in foam casting or powder metallurgy. According to
this method, metal powder is mixed with a foaming agent, for
example a gas. Gas pressure is derived by either a dispersed
particulate such as H.sub.2 from TiH.sub.2, high pressure generated
within an entrapped inert gas, or a gas injected into a liquid
metal. This mixture can then be extruded or cast into the
structural shape required. It is very difficult to control pore
size or orientation using these known techniques.
[0007] The powder metallurgical Fraunhofer-process is another
method used to create metallic foams. In this method, a foaming
agent is added to a metal powder that is then mixed. This mixture
can then be compacted or extruded into sheets or bars that can then
be formed into the component shape using conventional molding
techniques. Again, this process has little control of the pore size
or orientation. Furthermore, this method can be cost-prohibitive if
used to create geometrically complex parts due to the molds
required.
[0008] Recently, processes have been developed to attempt to
address the problem of lack of control of pore shape and
orientation. However, this process, referred to as the GASAR
process, involves the use of molten metals and the injection of
gases, and is therefore, technically complex and expensive.
Furthermore, the GASAR process allows the use of only one pore or
cell orientation in a component and the shapes of the components
are generally limited to plates, rods, and tubes.
[0009] Methods of making ceramic foams and cermet foams, which
contain both metallic and ceramic materials, encounter similar
difficulties. Such methods are extremely complex and do not produce
products with predictable quality or predictable porosity. Further,
prior cermet foams utilize a foaming agent, and thus both the shape
and the amount of porosity can not be controlled. As a result, it
is difficult to make foams that have consistent mechanical
properties. Additionally, the manufacture of complex shaped foams
using those methods is not feasible.
[0010] While others have developed processes for the fabrication of
metallic foam structures with oriented porosity, none of those
processes are capable of creating a combination of open and closed
cell porosity, nor are they capable of creating components directly
from CAD designs. Additionally, control of pore size and pore
orientation is difficult. Moreover, conventional processes do not
provide for the fabrication of integral structures with a metallic
foam skin in a cost-effective manner. Consequently, there is a need
for a fabrication process that can produce complex metallic foam
components with optimized dynamic mechanical properties in a
cost-effective manner.
[0011] Further, there is a need to fabricate metallic foam
structures that have both open and closed cell porosity. Open
porosity is characterized by the amount of surface area that is
accessible by a gas or liquid if the structure were to be immersed
in it, while closed porosity is the porosity in the structure not
accessible to a gas or liquid. The size, distribution, and aspect
ratio of close-celled porosity in a foamed material can have a
direct effect on its energy absorbing and blast amelioration
capability and other mechanical properties such as compressive
strength.
[0012] In addition, there is a need to fabricate foam structures
using ceramic materials or combinations of metallic and ceramic
materials. There is a need for a fabrication process that can
produce complex ceramic and cermet foam components with optimized
dynamic mechanical properties in a cost-effective manner.
SUMMARY OF THE INVENTION
[0013] The present invention overcomes the problems encountered
with conventional methods and compositions by providing efficient,
cost-effective processes for preparing complex metallic, ceramic
and cermet foam components with optimized dynamic mechanical
properties. More specifically, the invention provides methods and
compositions for metallic, ceramic and cermet foam components. In
particular, the present invention relates to the freeform
fabrication of metallic, ceramic, and cermet foams to form parts
having complex geometries and that demonstrate superior mechanical
properties and energy absorbing capacity.
[0014] Metallic foam components can be used in a wide range of
applications, for example, applications such as antenna masts and
fins, wings, exhaust ducts, electronic chassis components, and
ribbed heat diffusers for state of the art lighting fixtures.
Ceramic and cermet foams can be used in a wide variety of
applications including wear resistance applications, like high
temperature seals and similar applications.
[0015] Further, the present invention provides cost-effective
methods of producing ultra-lightweight structures directly from CAD
designs and offers the ability to manufacture complicated shape
prototypes with minimal post-processing steps. In addition, the
present invention provides methods and compositions for forming
foam structures that have both open and closed cell porosity. The
porous nature of the structures enhances their energy absorbing and
blast amelioration capability and other mechanical properties such
as compressive strength.
[0016] Accordingly, an object of the present invention is to
fabricate metallic, ceramic, and cermet foam components with
optimized mechanical properties in an efficient, cost-effective
manner.
[0017] Another object of the present invention is to fabricate
metallic, ceramic, and cermet foam components that display
increased energy absorbing and other mechanical properties based on
the foam having open and closed porosity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic describing the fibrillation of a
composite polymer blend through an extrusion orifice in accordance
with the present invention;
[0019] FIG. 2 is a cross-sectional scanning electron microscope
(SEM) image of unsintered Al--Cu foam fabricated according to this
method showing closed cell porosity;
[0020] FIG. 3 is a cross-sectional SEM image of an aluminum foam
fabricated according to this method showing closed cell porosity
and showing the larger pore sizes formed after exposure to a higher
temperature;
[0021] FIG. 4 is a plot showing normalized stress verses strain
rate for ALPORAS and ALULIGHT foams showing strain rate dependence
for the higher density ALPORAS closed cell foam;
[0022] FIG. 5 shows open porosity from a fully sintered aluminum
foam sample;
[0023] FIG. 6 is a longitudinal cross-sectional view of a fully
sintered aluminum foam sample sintered to 625.degree. C. in flowing
nitrogen, showing closed porosity;
[0024] FIG. 7 is a plot showing Split Hopkins Pressure Bar (SHPB)
compression test results at high strain rates for aluminum foam
fabricated according to this invention with L/D .about.2.0 and
density .about.1.25 g/cc;
[0025] FIG. 8 is a plot of SHPB compression tests conducted at high
strain rates as a function of density, showing the effect of
density;
[0026] FIG. 9 is a plot of SHPB compression tests conducted at
slower strain rates compared to samples in FIG. 8, which show the
effect of density;
[0027] FIG. 10 is a cross-sectional SEM image of a fully sintered
Si.sub.3N.sub.4 ceramic foam sample showing closed porosity;
and
[0028] FIG. 11 is a cross-sectional SEM image of a fully sintered
cermet (WCCo) foam sample showing closed porosity.
DETAILED DESCRIPTION OF INVENTION
[0029] The present invention relates to compositions and processes
that enable the manufacture of low cost, in situ, metallic,
ceramic, and cermet foam components having improved mechanical
properties such as energy absorption and specific stiffness. The
invention is not material specific, and it can be used with a wide
range of metals, alloys, ceramics, and combinations thereof.
Further, the present invention relates to foam composites having
oriented microstructure that are manufactured according to the
present method.
[0030] Generally, foam structures are fabricated from compositions
that include a major phase, which includes inorganic powder and a
binder. The inorganic powder of the major phase is selected as the
starting material for the foam bodies made in accordance with the
present invention. Generally, the powder is sinterable. Suitable
inorganic powders include metallic, ceramic, or combinations of
metallic and ceramic powders. The composition also includes a minor
phase, which is a compound that is generally immiscible with the
binder and generally non-reactive when mixed with the inorganic
powder. The desired object is fabricated from the composite blend
and finished to create the foam object.
[0031] Any metal or metal alloy powder can be used in this
invention. Metals and alloys that can be used include, but are not
limited to, Al, Cu, Mg, Sn, Ti, Zn, Co, Ni, Mo, Nb, and the like,
and combinations including alloys thereof. Composites and
pre-alloyed powders can also be used, including stainless steel,
Ti64 (Ti-6Al-4V), and numerous others. A preferred alloy contains
aluminum powder with Cu, Si, Sn, and/or Mg, such as a composition
containing aluminum and 6.39 wt % Mg and 6.72 wt % Sn. Another
preferred alloy blend contains aluminum and 4 wt % Cu alloy. Use of
a eutectic alloy composition enhances the sinterability of the
composition by making it easier to break the oxide layer on top of
the aluminum powder. Additionally, the sintering temperature of the
composition is reduced. Furthermore, the eutectic compositions
provide higher strength aluminum alloys compared to pure aluminum
thus, making it possible to obtain metallic foam components with
better energy absorption capability.
[0032] Further, the inorganic powder can also be ceramic powder.
Ceramic powders that can be used include, but are not limited to
any carbide, nitride or oxide compound such as SiC,
Si.sub.3N.sub.4, alumina, ZrO.sub.2, ZrC, HfC, Si--Al--O--N,
WC--Co, and the like, and combinations thereof.
[0033] Combinations of metallic and ceramic powder may be used to
fabricate cermet foams. As used herein, "cermet" refers to
compositions that include both ceramic and metallic powders. Any
thermodynamically compatible metallic and ceramic powders,
including the previously listed ceramic and metallic powders, can
be combined to fabricate cermet foams.
[0034] The concentrations of the inorganic powders typically are
between about 55% to about 96% by weight of the foam composition to
provide final bodies having generally appropriate structural
strength. Lower concentrations may be used, however, as the powder
concentration decrease, it is expected that structural strength
also may decrease. More typically, powder concentrations are
between about 70% to about 90% by weight of the foam composition,
and even more typically between about 75% to about 85% by weight of
the foam composition.
[0035] Particle sizes for ceramic powders are generally between
about 0.1 to about 45 .mu.m, with preferred particle sizes between
about 1 to about 10 .mu.m. Particle sizes for metallic powders are
generally between about 1 to about 45 .mu.m, with preferred
particle sizes between about 20 to about 45 .mu.m. While generally
any particle size may be used, smaller particle sizes provide
enhanced finishing of the formed structures by limiting the amount
of shrinkage that may occur during binder burnout and
sintering.
[0036] The binder of the of the major phase functions to suspend
the metallic, ceramic or cermet powder and provide a flowable
composition. Preferably, thermoplastic and thermoset polymer
binders may be used, and are selected based on several factors,
including the powder used, the desired processing to create and
finish the desired object, etc. The amount of polymer binder needed
will depend n the density of the ceramic, metallic, or cermet
powder. Preferably, the mixture contains binder in an amount from
about 40% to about 60% by volume, and more preferably from about
45% to about 50% by volume based on the foam composition. The
polymer binder may be selected to be compatible with use in an
injection molding process, extrusion, or gel casting processes.
[0037] Thermoplastic polymer binders that can be used include, but
are not limited to, PMMA (polymethyl methacrylate), EVA (ethyl
vinyl acrylate), EEA (polyethylene ethacrylate), PEOx
(poly-2-ethyl-2-oxazoline), PEG (polyethylene glycol), polystyrene,
microcellulose, and the like, and combinations thereof. Thermoset
polymer binders that can be used include, but are not limited to,
BLO (butrylactone, which is commercially available from Aldrich
Chemical Company in Milwaukee, Wis.), HODA (hexanedioldiacrylate,
which is also commercially available from Aldrich Chemical Company
in Milwaukee, Wis.), and the like and combinations thereof.
[0038] The minor phase compound is selected to be removable during
the finishing stages to provide pores in the structure. The minor
phase is mixed with the major phase, and when subsequently removed,
correspondingly shaped pores remain within the structure. Blending
of the major and minor phases need not be uniform. The minor phase
may be present in the foam composition as a plurality of discrete
droplets or other regular or irregular shapes of either generally
uniform or varying sizes. The minor phase compound can be any
compound that remains generally immiscible with the polymer binder
and metallic, ceramic, or cermet powder when blended together.
Further, it should be generally non-reactive with the metallic,
ceramic, or cermet powder.
[0039] The minor phase compound can be a polymer binder selected
from those polymers binders previously listed, as long as it is
generally not miscible with the first polymer binder used in the
blend. The minor phase compound also can be a polymer-immiscible
material including, but not limited to, camphor, xylene, butyl
oleate, and foaming agents such as carbamide (urea), ammonium
hydrogen carbonate, and titanium dihydride. Examples of
combinations of polymer binders that can be used include
polystyrene and PEOx, microcellulose and PMMA, and PMMA and xylene.
Examples of major and minor phases for an aluminum foam composition
include PEOx with polystryene, and PMMA with titanium dihydride.
Examples for a stainless steel foam composition include PPMA with
polyethylene glycol and PMMA with ammonium hydrogen carbonate.
Further, the minor phase compound can be microspheres, or
microballoon particles. The microspheres can be polymer or ceramic
microspheres. For example, polyacryonitrile microballoon particles
such as Micropearl.TM. from Matsumoto Yushi-Seiyaku Co., which is
available in various grades such as 30, 50, 80, 100, and 120 .mu.m
can be used. Ceramic microspheres, such as silica microspheres from
PQ Corporation, SiO.sub.2 or A.sub.2O.sub.3 also can be used.
Generally, ceramic microspheres are available as hollow spheres,
where the hollow center of the sphere becomes a pore within the
structure without additional processing such as burn out of a
polymer. Any suitable size of microspheres, and combinations of
sizes and types of microspheres can be used to obtain the desired
pore size and amount of closed porosity.
[0040] Typically, the major phase is present in an amount in excess
of the minor phase, with the ratio of the major and minor phase
based at least in part on the components of the blend. For example,
with certain blends it may be desirable to use approximately twice
as much or more (by volume) of the major phase. With other
compositions, such as where carbamide, titanium dihydride or
ammonium hydrogen carbonate are used as the minor phase, a smaller
amount is needed for the minor phase, typically less than 1% by
volume of the total blend. When microspheres are used as the minor
phase, the amount used depends on the amount and size of porosity
desired in the final structure, which is typically between about 20
to about 70 volume percent of the component part depending on the
application, though other porosities are possible. By way of
example, to achieve about 20% porosity of the finished structure,
about 0.0085 g of microspheres per gram of slurry is required.
[0041] Depending on the major and minor phase materials chosen for
the blend, additional processing aids may be added. When a
thermoplastic binder system is used, a compatibilizer or surfactant
may be added to achieve high solids loading, to disperse the powder
more effectively, and to reduce the viscosity of the mixture.
Various compatibilizers may be used including, polyethylene glycol,
stearic acid, xylene, and butyl oleate, or other additives like Al
3 wax and the like and combinations thereof may be used. In the
case of a thermoset binder system, a dispersant is typically added
in order to achieve high solids loading and to reduce the viscosity
of the mixture. It also acts as a compatiblizer for the various
components and reduces the viscosity of the composition. Various
dispersants may be used including, but not limited to,
organophosphate esters such as MAPHOS.TM. 8135, which is
commercially available from BASF Corporation in Folcroft, Pa., and
phosphate esters, such as PS-2 and the like.
[0042] A sintering aid can also be added. For example, ceramics
typically have very high melting points, and the sintering aid is
typically chosen based on the melting point of the ceramic
material. For example, when the composition includes SiC or
Si.sub.3N.sub.4, sintering aids including, but not limited to,
yttrium, alumina, ceria (Ce.sub.2O.sub.3), other rare earth oxides,
and the like, or combinations thereof may be used. Further, when
the composition includes WC, alumina and similar materials may be
used as a sintering aid. Sintering aids that can be used in cermet
formulations include, but are not limited to metals like Ni, Mo,
Nb, and the like, and combinations thereof.
[0043] Furthermore, other additives to enhance the mechanical
properties of the desired object may be added. The additive is
typically chosen to be thermodynamically compatible with the blend.
For example, to increase the wear resistance, MoC, SiO.sub.2, MoC,
NiC, WC, HfC and FeC, and the like, and combinations thereof may be
added.
[0044] The composite blends are prepared by melting the binder and
mixing with the inorganic powder and the minor phase. A high shear
mixer, such as a Brabender high-shear mixer, may be used. In order
to ensure desired homogeneity of the blend, the material may be
rebatched (material is removed from the mixing head, the mixing
head is cleaned, and the material is put back into the head for
further mixing). Upon completion of the rebatching, the blend is
allowed to cool and then taken out of the mixing head. In one
aspect, the binder and powder are mixed to provide a flowable
composition. Mixing of the minor phase compound and flowable
composition is incomplete so that the minor phase compound remains
as discrete droplets or portions within the flowable composition
and does not become uniformly blended with the powder.
[0045] To form the structures, the composite blend can be processed
using any known process, such as, for example, low-pressure
injection molding for thermoplastic binders, gel casting for
thermoset binders, or ribbon or filament deposition processes. In
one embodiment, an extrusion freeform fabrication (EFF) process for
forming three-dimensional objects is used to provide the foam
structures. An EFF process can be adapted to allow for rapid
fabrication of functional components from the composite blends of
the present invention. Such a process allows for the sequential
deposition of multiple layers of the compositions to form
complex-shaped structures, as desired. Preferably, the process
equipment includes a fabrication modeler fitted with a
high-pressure extrusion head to allow for extrusion of the highly
viscous polymer systems of the present invention. For example, any
commercially available rapid prototyping system such as a Stratasys
Fused Deposition Modeling Device, can be adapted to use a
high-pressure extrusion head, which assists in extruding the highly
viscous foam compositions. Other methods that include high-pressure
extrusion or otherwise allow the typically viscous composite blend
to be deposited in a predetermined manner to form the desired
structures may be used.
[0046] Generally, a pre-formed feed rod is prepared from the
composition and passed to the extrusion apparatus. The feed rod is
passed through an extrusion apparatus where an extrusion head of
the apparatus deposits the extruded composition onto a work or
support surface. Alternatively, the components of the composition,
including polymer materials and additives, can be passed directly
to the apparatus for a continuous process. The ribbons or filaments
of extruded material generally are deposited layer upon layer onto
the work or support surface in a predetermined pattern to form an
object of the desired shape and size and having the desired
porosity characteristics. Preferably, the extruded material is
deposited so that the longitudinal axis of the extruded material is
generally parallel to the work surface.
[0047] Depending upon the particular processing conditions
employed, the minor phase of the polymer binder blend may become
deformed when subjected to shear conditions. Without wishing to be
bound by any theories, the amount of deformation experienced by
these droplets is a function of the shear stress rate imparted to
the blend, the viscosity of the individual polymers constituting
the blend, and the diameter of the minor phase material. Therefore,
the high-pressure extruder head is preferred in order to be able to
extrude high polymer binder content viscous systems while being
able to maintain a predetermined alignment of the minor-phase
fibril in the extrusion direction.
[0048] A relationship detailing the elongation of a minor phase
droplet in a polymer blend when subjected to shear stress is as
follows:
.epsilon.(t)=.sub..chi.I(t)(19.lambda.+16)/(16.lambda.+16),
where
[0049] .epsilon.(t): droplet elongation as a function of time
[0050] .lambda.: ratio of major phase to minor phase of apparent
polymer shear viscosities
[0051] .chi.: .sigma.d/.gamma.; shear stress(.sigma.)*droplet
diameter (d)/interfacial tension (.gamma.)
[0052] I(t): time dependent integral:
1-exp(-40.epsilon.'t/19.lambda..sub.- .chi.) , where .epsilon.' is
the deformation rate
[0053] As this relationship demonstrates, these factors are
strongly dependent upon blend extrusion conditions. The polymer
component viscosity and interfacial tensions are influenced by
extrusion temperature, while shear stress imparted to the blend
increases with extrusion pressure and decreased extruder orifice
diameter. Initial (quiescent) droplet diameter of the minor blend
component is dependent upon the relative concentration of the minor
phase in the blend. The morphology of these droplets is therefore
strongly influenced by the amount of shear imparted to the
polymer.
[0054] The minor phase is typically present as discrete droplets or
particles dispersed in the major phase. Such a dispersion is a type
of microstructure that is typically encountered in blends composed
of two immiscible phases, where the minor phase adopts a spherical
morphology to minimize its surface area and energy. During
extrusion, the minor phase may become aligned, e.g. elongated and
positioned, in the direction of extrusion. When a blend is
initially stressed, the spherical droplets of the minor phase
become elongated into an ellipsoidal geometry. Increased stress
causes the ellipsoids to become oriented with their major axes
generally parallel to the polymer extrusion direction. These
ellipsoids become elongated into long continuous fibrils which are
oriented generally parallel to the flow direction. The transition
between spherical minor phase to ellipsoids and subsequent
fibrillation while extruding a composite blend through a nozzle is
illustrated in FIG. 1.
[0055] The minor phase undergoes fibrillation when subjected to
high shear extrusion through the high-pressure extrusion head.
Preferably, this shear force aligns the minor phase along the
extrusion direction as the feed rod passes through the orifice of
the high-pressure extrusion head. Because the high-pressure
extrusion head allows the orientations of the fibrils to be
manipulated, a tailored microstructure with predetermined material
properties will result. Accordingly, the extrusion head provides
the ability to tailor the microstructure.
[0056] Depending on the materials used, extrusion temperatures can
vary widely, for example from about 70.degree. C. to about
400.degree. C. The extrusion temperature is dependent at least in
part on the melting temperature of the binder system and the
particle size distribution of the powder. For example, for an EEA
binder, the composite blend can typically be extruded at
temperatures between about 125-150.degree. C., though others blends
may require different temperatures.
[0057] After passing through the high-pressure extrusion head, the
extruded composite blend is deposited in a controlled manner onto a
working surface. If desired, the extruded composite blend may then
be formed into parts forming a final component directly from CAD
designs. It is thus desirable that the system has hardware capable
of processing CAD drawings so that the form, such as a mesh-like
structure, can be built by a rapid prototyping process. The
extruded composite blend can be used to fabricate parts in a
layer-wise fashion by the sequential stacking of discrete extruded
raw material layers upon each other until the final component part
is formed. Each layer has a geometry corresponding to a horizontal
cross section of the desired prototype. In this manner,
geometrically complex parts are manufactured with complex and
varying orientations of microstructure. Preferably, the composite
blend is deposited in such a way as to provide a mesh-like or
similar structure having the desired overall geometry. The
mesh-like formation of the structure, as compared to a solid
structure, provides an additional level of porosity in the
structure.
[0058] In another embodiment of this invention, inorganic powders
and polymer binder are mixed to create a slurry. A ball mill or
similar mixing apparatus can be used to mix the ingredients to
create the slurry. When a polymer binder such as HODA or BLO is
used, the inorganic powder is preferably added in batches to the
slurry. Typically, a dispersant is added, and a sintering aid may
also be added depending on the materials used. The minor phase
compound is also added to the slurry. The desired component part or
structure can be created from the slurry by any suitable method,
including gel casting, injection molding, and the like. Additional
processing aids also can be added to the slurry depending on the
selected fabrication method. For example if gel casting in a mold
is used, a sufficient amount of a gelling agent or initiator to gel
or cure the mixture is added to the mixture before it is poured
into the mold. After the foam composition gels in the mold, the
gelled part can be removed from the mold for binder burnout and
sintering. Material densification and consolidation of the part is
achieved through binder burnout and sintering or similar
processes.
[0059] Generally, the closed porosity in the structures is formed
in the component part generally during the finishing processes,
which involve heating the object to remove at least a part of the
minor phase leaving internal voids in the structure to provide the
closed porosity as seen in FIG. 2. As previously explained, closed
porosity in a structure is not accessible to the outside and is
completely enclosed in the final structure. Modifications in the
burnout schedule may be desired to further optimize the pore-size.
For metallic and cermet foams, it may be preferable to perform the
binder burnout in either flowing nitrogen or flowing nitrogen with
about 5% hydrogen, and the components may be supported in a powder
bed of titanium sponge held in a ceramic crucible. Preferably, the
crucible is made of alumina, though other crucible materials may be
used.
[0060] The temperature required for binder burnout varies with the
materials used. Generally, ceramic and cermet structures require
higher temperatures but shorter time periods than temperatures
required for metallic foams. By way of example, PMMA requires
temperatures in the range of about 430.degree. C. to about
450.degree. C. while EVA and EEA type polymers require temperatures
in the range of about 600.degree. C. to about 630.degree. C.
[0061] Further, depending on the compositions, a cycling burnout
schedule may be used to achieve desired results and to increase
closed porosity. Cycling binder burnout schedules involve
incremental raises in the temperature, each followed by holding at
a specific temperature. Preferably, cycling burnout schedules with
incremental temperature changes and holding at certain
temperatures, as opposed to heating at one constant temperature,
are used. The cycling burnout schedule, including the amount of
incremental temperature increase as well as the maximum temperature
required, will depend on the particular materials used.
[0062] By way of example, for metallic foams, the amount of closed
porosity in the structure may increase with holding the burnout
temperature at about 150.degree. C. to about 190.degree. C. This
temperature is dependent on the temperature stability of the minor
phase. Burnout cycles are typically completed by about 450.degree.
C., which is well below the temperature required for the sintering
temperature of the foam components. Further, pore size increases
with increasing time of exposure and temperature in the range of
about 150.degree. C. to about 190.degree. C. This is shown by
comparing the size of the pores in FIGS. 2 and 3. Compared to the
porosity size in FIG. 2, which was heated at about 150.degree. C.,
holding the temperature at 190.degree. C. for the same time
duration yielded larger porosity, as shown in FIG. 3. Similar
effects would likely be observed with a longer hold time at the
same temperature.
[0063] The binder burnout process may be followed by furnace
cooling. After furnace cooling, the component can be sintered to
fully consolidate the component. The sintering temperature depends
on the type of metallic, ceramic, or cermet powder used as well as
whether a sintering aid was used. Typically, the sintering
temperatures necessary for ceramic and cermet foams are higher than
those necessary for metallic foams. By way of example, for SiC
foams the sintering temperature is typically about 1850.degree. C.,
for Si.sub.3N.sub.4 foams the sintering temperature is typically
about 1750.degree. C., for aluminum foams the sintering temperature
is typically about 625.degree. C., for aluminum alloys the
sintering temperature is typically about 625.degree. C. to about
650.degree. C., for copper alloys the sintering temperature is
typically about 850.degree. C. to about 950.degree. C., and for
stainless steel the sintering temperature is typically about
1250.degree. C. to about 1350.degree. C.
[0064] A predetermined degree of closed and open porosity may be
achieved through this invention depending on the materials and
specific methods used. The porosity levels and the pore sizes can
be controlled by the powders, binders and minor phase components
chosen and are related to the hold temperature during the binder
burnout stage. Foam structures fabricated according to this
invention typically have closed porosity up to about 70%, and
preferably between about 20% to about 70%. Although porosity levels
greater than about 70% can be achieved, such porosity levels may
begin to adversely impact the strength and durability of the
structures. Preferably, the size of the closed pores ranges from
about 30 .mu.m to about 150 .mu.m, though larger and smaller pores
may be fabricated. For example, if microspheres are used for the
minor phase, the size of the resultant pores is related to the size
of the microspheres used. Thus, by way of example, 100 .mu.m
microspheres will typically create closed pores about 100 .mu.m
after burnout and sintering of the object.
[0065] Foam structures having spherical closed porosity with higher
aspect ratios can be fabricated. Spherical closed porosity provides
better energy absorption capability than currently available foams
with irregular or hexagonal shaped open cells. Closed porosity
provides improved energy absorption capabilities as compared to
open porosity. This is closely related to the aspect ratio of the
cells themselves, the aspect ratio being the cell wall thickness
divided by the length of the cell. Circular cells have the
capability to absorb more impact energy than hexagonal cells that
are intrinsically unstable. Spheroidal cell walls are thicker on
average, and the reduced section thickness is too short to lead to
compressive instability. In fact, spheroidal walls may experience
some plastic deformation prior to cell wall rupture. The size,
distribution, and aspect ratio of closed-celled porosity in a
foamed material can have a direct effect on its energy absorbing
and blast amelioration capability and other mechanical properties
such as compressive strength.
[0066] The amount of open porosity desired is dependent on the
specific application. For example, it may be desirable in certain
applications for ceramic foams to have very low open porosity.
Further, it may be desirable in other applications for metallic
foams to have both open and closed porosity. The structures
preferably have open porosity ranging from about 0% to about 70%.
For many applications and to provide the benefits of open porosity
while maintaining the structural integrity of the object, the open
porosity should typically be between about 30% to about 70%, with
pore sizes ranging between about 100 .mu.m to about 1000 .mu.m or
larger.
[0067] Without wishing to be bound by any theories, it is believed
that higher density and higher specific strength closed-cell foam
materials provide improved energy absorption as compared to
open-celled foams or low-density closed-cell foams. The increased
energy absorption capability is directly related to the increases
in plateau stress and strain. This effect is believed to be due to
the spheroidal cell walls being thicker on average and the reduced
section thickness being too short to lead to cell wall buckling.
For the same density, a foam material with a higher amount of
closed-cell porosity will also generally have a higher compressive
strength compared to an open-cell porous material, which is much
weaker than a closed-cell foam material with the same amount of
porosity. For example, the strain rate dependence of yield strength
for closed cell ALPORAS foam is shown in FIG. 4. In this figure,
the y-axis in the plot is the normalized plateau stress. It is
normalized based on the relative density of the specific foams. The
other types of foams shown in FIG. 4 are of the open porosity
variety.
[0068] The following examples are intended to illustrate the
present invention and should not be construed as in any way
limiting or restricting the scope of the present invention. The
following examples illustrate formulations of the composites that
can be used in preparing metallic, ceramic, and cermet foams.
Further, some of the examples illustrate burnout schedules that can
be used to achieve sufficient porosity.
EXAMPLES
Examples 1
[0069] This example illustrates a blend formulation that can be
used to make metallic foam. As shown in Table I, aluminum powder
was blended with copper powder to produce an Al-4 wt % Cu alloy
blend. The copper powder was added to the aluminum powder to create
an alloy composition that would be more readily sinterable than
pure aluminum alone. The aluminum and copper powders were similar
in size. The Al and Cu powders were approximately 45 .mu.m size
particles, with an average particle size of 10 .mu.m. The polymer
(water-soluble) is a multi-component mixture. It was first melted
in a Brabender high-shear mixer and the aluminum-copper powder
mixture was then added as carefully as possible.
1 TABLE I Batch Size (cc): 42 Batch Temperature (C.): 150 Batch
Speed (rpm): 60 Density Weight (W) Volume (V) Actual Actual g/cc %
% V W (g) Aluminum 2.702 95.5 0.542 22.78 61.54 Copper 8.96 4.5
0.008 0.32 2.90 PMMA 1.14 0.450 18.90 21.94 Total 100 1.000 42.00
86.38
[0070] After compressing the composite blend into feed rods, they
were extruded at temperatures of about the 225-250.degree. C. Next,
the binder burn-out was performed using either flowing nitrogen or
flowing nitrogen with 5% hydrogen, followed by furnace cooling.
Using this formulation, the cycling binder burnout schedule in
Table II was used.
2TABLE II Temperature Temperature Range (.degree. C.) Increase
Increment Total Time (hr) 30-270 30.degree. C. 8 Hold -- 2 270-400
10.degree. C. 13 Hold -- 1 400-430 30.degree. C. 1 Hold -- 1
[0071] FIG. 2 shows a cross-sectional scanning electron microscope
image of an aluminum foam with closed porosity fabricated according
to this invention.
Example 2
[0072] This example illustrates another blend formulation that can
be used to make metallic foam. As shown in Table III, aluminum was
used as the primary metallic powder with small amounts of Mg and Sn
added to the aluminum in an effort to reduce oxide layers formation
on the metallic powder. In this example, the composite blend
contained 8 wt % of Mg and 8 wt % of Sn. Additionally, Mg and Sn
form a eutectic alloy with Al well below the maximum binder burnout
temperature, thus allowing the foam material sufficient structural
rigidity to be handled between the binder burnout and sintering
steps.
[0073] This formulation used the polymethylmethacrylate (PMMA) as
the polymer binder. Although PMMA has a relatively high viscosity,
it was used as the polymer binder because it has clean burn-off
characteristics. In this example, the xylene also served to reduce
the viscosity of the PMMA.
3TABLE III Material Density (gm/cc) Volume % Weight % Weight (gm)
Al 2.702 43.75 67.1 49.65 Sn 7.31 1.25 6.2 4.6053 Mg 1.738 5.00 4.7
3.46731 PMMA 1.1 35.25 22.0 16.2855 Xylene 0.86 14.75 7.2 5.3277
Totals 100.00 100% 74.01
[0074] After compressing the composite blend into feed rods, they
were extruded at temperatures of about 225-250.degree. C. The
extrusion process was observed to watch for breakdown of the binder
system in this temperature range. Next, the binder burn-out was
performed using either flowing nitrogen or flowing nitrogen with 5%
hydrogen, followed by furnace cooling. Using this formulation, the
cycling binder burnout schedule in Example 1 was successfully used.
FIG. 5 shows the open porosity of a fully sintered aluminum foam
component with the composition in Table III using the binder
burnout schedule from Table II after sintering at 625.degree. C. in
flowing nitrogen.
Example 3
[0075] Metallic foam components were made using the formulation in
Table IV. The major binder phase was PMMA and the minor binder
phase was butyl oleate. The same general procedure as in the
previous examples was used with this formulation.
4TABLE IV Material Density (gm/cc) Volume % Weight % Weight (gm) Al
2.702 43.75 62.6 49.65 Sn 7.31 1.25 4.84 3.84 Mg 1.738 5.00 4.60
3.65 PMMA 1.1 40.00 23.29 18.48 Al 3 wax 0.92 5.00 2.43 1.93 Butyl
Oleate 0.855 5.00 2.27 1.8 Totals 100.00 100% 79.35
[0076] Further, this formulation permitted the use of enhanced
extrusion forces and the use of a smaller extrusion tip with a
0.016" extrusion nozzle. Prior to extrusion, the feedrods were heat
treated at 100.degree. C. for 2 hours. This formulation was
successfully extruded at 170.degree. C. and 275-300% flow. This
formulation and extrusion conditions provided a smoother surface
finish after extrusion.
[0077] The burnout schedule, shown below in Table V, was used for
this formulation. The hold times listed in Table V can be adjusted
in proportion to the dimensions of the sample. The burnout was
performed in either flowing nitrogen or flowing nitrogen with 5%
hydrogen, followed by cooling. Depending on the shape and
configuration of the sample, it may be necessary to pack the
samples during both the binder burnout and sintering stages.
Titanium sponges and alumina crucibles can be used to pack the
samples.
5TABLE V Temperature Temperature Range (.degree. C.) Increase
Increment Total Time (hr) 30-170 20.degree. C. 7 Hold -- 2 170-270
25.degree. C. 4 Hold -- 2 270-400 10.degree. C. 13 Hold -- 1
400-430 15.degree. C. 2 Hold -- 1
[0078] FIG. 6 shows a longitudinal cross-sectional view of a fully
sintered aluminum foam sample according to this example. Dynamic
Split Hopkins Pressure Bar (SHPB) strain rate test results obtained
from this formulation are similar to those shown in FIGS. 7 through
9.
Example 4
[0079] This example illustrates a blend formulation for a ceramic
foam as shown in Table VI.
6TABLE VI Constituent Density (g/cc) Vol % (Partial den) Wt %
Si.sub.3N.sub.4 3.21 46.46 1.48 68.95 Y.sub.2O.sub.3 3.26 1.00 1.51
1.51 MoC 9.18 0.04 4.24 0.15 SiO.sub.2 2.65 0.00 1.23 0.00
Al.sub.2O.sub.3 3.97 2.50 1.84 4.59 BLO 1.12 26.50 0.52 13.72 HODA
1.01 21.00 0.47 9.81 MAPHOS 8135 1.20 0.89 0.55 0.49 PS-2 1.05 1.61
0.49 0.78 Totals 2.16 100.00 100.00
[0080] For this formulation, the Si.sub.3N.sub.4 was first combined
with the Y.sub.2O.sub.3 and the Al.sub.2O.sub.3, which are the
sintering aids, as well as with the MoC and the SiO.sub.2, which
were added to increase the wear resistance. The mixture was milled
for between 48 to 96 hours in an ethanol medium. The ethanol was
then evaporated, and the powder was dried. The powder was then
divided into 5-6 equal batches, and then it was added to the
BLO-HODA mixture with ceramic milling media, and mixed in a
container that was rolled on a rolling mill to ensure uniformity of
mixing of the contents. After milling for approximately 2-3 hours,
the surfactants, PS-2 and MAPHOS 8135, were added to reduce the
viscosity of the composition. After further milling for
approximately 1 hour, the microballoon particles were added. The
gelling agents (initiator and catalyst) were added.
[0081] The slurry was then poured into the mold. Further vacuum was
applied to remove any air trapped inside to gel the slurry. The
gelled part was then removed from the mold. After binder burnout
and sintering, an object with closed porosity was fabricated, as
shown in FIG. 10.
Example 5
[0082] This example illustrates a blend formulation for a cermet
foam as shown in Table VII. The same general procedure as in the
previous example was used with this formulation.
7TABLE VII Constituent Density (g/cc) Vol % (Partial den) Wt %
WC/Co 6% 14.960 55.00 1.72 94.47 BLO 1.12 22.00 0.13 2.83 HODA 1.01
20.00 0.12 2.32 MAPHOS 8135 1.20 1.00 0.14 0.14 PS-2 1.05 2.00 0.12
0.24 Totals 8.71 100.00 100.00
[0083] The WC--Co powder was divided into 5-6 equal batches. The
powder was added to the BLO-HODA mixture with ceramic milling
media, and mixed in a container that was rolled on a rolling mill
to ensure uniformity of mixing of the contents. After milling for
approximately 2-3 hours, the surfactants such as PS-2 and MAPHOS
8135 were added to reduce the viscosity of the composition. After
further milling for approximately 1 hour, microsphere particles
were added. The gelling agents (initiator and catalyst) were
added.
[0084] The slurry with the gelling agents was poured into the mold.
After binder burnout and sintering, a cermet foam object with
closed porosity was obtained, as shown in FIG. 11.
[0085] Numerous modifications and variations may be made in the
techniques and structures described and illustrated herein without
departing from the spirit and scope of the present invention. Thus,
modifications and variations in the practice of the invention will
be apparent to those skilled in the art upon consideration of the
foregoing detailed description of the invention. Although preferred
embodiments have been described above and illustrated in the
accompanying drawings, there is no intent to limit the scope of the
invention to these or other particular embodiments. Consequently,
any such modifications and variations are intended to be included
within the scope of the following claims.
* * * * *